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3.02.2012

Acidic Europa May Eat Away at Chances for Life

Oxidants from the surface of Jupiter's moon Europa's might react with
sulfides and other compounds in the moon's ocean, generating acids.
This could make the ocean too acidic to support life as we know it.

Europa. Credit: NASA

The ocean underneath the icy shell of Jupiter's moon Europa could be too
acid to support life, due to compounds that may regularly migrate
downward from its surface, researchers find.

Europa, which is roughly the size of Earth's moon, could possess an ocean
about 100 miles deep (160 kilometers). This ocean is overlain by an icy
crust of unknown thickness, although some estimates are that it could
be only a few miles thick.

Since there is life virtually wherever there is liquid water on Earth,
for many years scientists have entertained the notion that this Jovian
moon could support extraterrestrials. Recent findings even suggest its ocean could be loaded with oxygen,
enough to support millions of tons worth of marine life like the kinds
that exist on Earth. Researchers have proposed missions to penetrate Europa's outer shell to look for life in its ocean, although others have suggested that fossils of marine life
on Europa could be available right on the surface for prospectors to
find, given how water apparently regularly gets pushed up from below.

However, chemicals found on the surface of Europa might jeopardize any
chances of life evolving there, scientists find. The resulting level of
acidity in its ocean "is probably not friendly to life — it ends up
messing with things like membrane development, and it could be hard
building the large-scale organic polymers," said researcher Matthew
Pasek, an astrobiologist at the University of South Florida.

Model of Europa's interior. The moon is
thought to have a metallic core surrounded by a rocky interior, and then
a global ocean on top of that surrounded by a shell of water ice.
Credit: NASA

However, chemicals found on the surface of Europa might jeopardize any
chances of life evolving there, scientists find. The resulting level of
acidity in its ocean "is probably not friendly to life — it ends up
messing with things like membrane development, and it could be hard
building the large-scale organic polymers," said researcher Matthew
Pasek, an astrobiologist at the University of South Florida.

The compounds in question are oxidants, which are capable of receiving
electrons from other compounds. These are usually rare in the solar
system because of the abundance of chemicals known as reductants such as
hydrogen and carbon, which react quickly with oxidants to form oxides
such as water and carbon dioxide. Europa happens to be rich in strong
oxidants such as oxygen and hydrogen peroxide which are created by the
irradiation of its icy crust by high-energy particles from Jupiter.

The oxidants on Europa's surface are likely carried downward in
potentially substantial quantities by the same churning that causes
water to rise from below. Oxidants could be of great use to any life in
Europa's ocean — for example, oxygen was pivotal to how complex life
evolved on Earth.

However, oxidants from Europa's surface might react with sulfides and
other compounds in this moon's ocean before life could nab it,
generating sulfuric and other acids, investigators said. If this has
occurred for just about half of Europa's lifetime, not only would such a
process rob the ocean of life-supporting oxidants, but it could become
relatively corrosive, with a pH of about 2.6, "about the same as your
average soft drink," Pasek said.

This level of acidity would be a significant challenge for life,
unless organisms were to consume or sequester oxidants fast enough to
ameliorate the acidification, researchers said. The ecosystem would need
to evolve quickly to meet this crisis, with oxygen metabolisms and acid
tolerance developing in only about 50 million years to handle the
acidification.

Any surviving ecosystem in Europa's ocean might be analogous to microbes
found in acid mine drainage on Earth, such as the bright red Río Tinto
river in Spain. The dominant microbes found there are acid-loving
"acidophiles" that depend on iron and sulfide as sources of metabolic
energy.

"The microbes there have figured out ways of fighting their acidic
environment," Pasek said. "If life did that on Europa, Ganymede, and
maybe even Mars, that might have been quite advantageous."

Others have questioned whether or not rock in Europa's seabed might
actually neutralize the effects of this acidity. Pasek does not think
this is likely — even if such minerals were present, there is probably
not enough of it exposed to reduce acidity by much, he said.

The calcium-based materials that bones and shells on Earth are made from
might dissolve pretty readily in such an acidic environment. However,
"one of the interesting possibilities is that they might have use blue
phosphates as their bone material instead to evolve large organisms,"
Pasek said. "If you have iron phosphates, you make a pretty blue mineral
called vivianite."